ABSTRACT
In the year 1923, Rutherford had discovered the proton but, in his subsequent experiments, the atomic number was consistently less than the atomic mass. is could not be due to the number of electrons, so Rutherford hypothesized that there was another type of particle in the atomic nucleus-one with mass but no charge. In another development in 1925, physicists proposed the idea of nuclear spin to explain the Zeeman eect (shifts in atomic energy levels in a magnetic eld), but this did not seem to t the prevailing model for the atomic nucleus, believed to contain just protons and electrons. e additional hint of the neutron’s existence came in 1930, when Walther Bothe and H. Becker found that when alpha radiation fell on elements like lithium and boron a new form of radiation was emitted. Initially, this radiation was believed to be a type of gamma radiation, but it was more penetrating than any known gamma radiation. Work by Irene Joliot-Curie and Frederic Joliot in 1932, though not disproving the gamma radiation hypothesis, did not particularly support it either.
